Tuesday, October 21, 2014

a) It was a meteorite impact and the resulting environmental crises.
b) No, it was the Deccan basalt eruptions and the resulting environmental crises.
c) It was both, the meteorite impact and the eruptions.

The two mechanisms were distinct. One, a calamity from space and the other a gigantic eruption whose cause was from deep within the earth.

Now, it seems they may be more intimately linked. The meteorite impact may have triggered the largest pulse of the Deccan volcanic eruptions. Be careful here. The impact did not initiate Deccan volcanism. That was caused by India rifting away from Madagascar (88 mya) and Seychelles (66 mya). The rifting and an unusually hot mantle underneath resulted in copious amounts of melt being generated in the mantle which found its way to the surface via the great tensional cracks formed when continents separate. The impact though may have resulted in increasing the permeability of the mantle underneath thus making it possible for larger amounts of magma to make its way to the surface.

New constraints on the timing of the Cretaceous-Paleogene (K-Pg) mass extinction and the Chicxulub impact, together with a particularly voluminous and apparently brief eruptive pulse toward the end of the “main-stage” eruptions of the Deccan continental flood basalt province, suggest that these three events may have occurred within less than about a hundred thousand years of each other. Partial melting induced by the Chicxulub event does not provide an energetically-plausible explanation for this coincidence, and both geochronologic and magnetic-polarity data show that Deccan volcanism was underway well before Chicxulub/K-Pg time. However, historical data document that eruptions from existing volcanic systems can be triggered by earthquakes. Seismic modeling of the ground motion due to the Chicxulub impact suggests that the impact could have generated seismic energy densities of order 0.1-1.0 J/m3 throughout the upper ~200 km of the Earth’s mantle, sufficient to trigger volcanic eruptions worldwide based upon comparison with historical examples. Triggering may have been caused by a transient increase in the effective permeability of the existing deep magmatic system beneath the Deccan province, or mantle plume “head.” It is therefore reasonable to hypothesize that the Chicxulub impact might have triggered the enormous Poladpur, Ambenali, and Mahabaleshwar (Wai sub-group) lava flows that account for >70% of the Deccan Traps main-stage eruptions. This hypothesis is consistent with independent stratigraphic, geochronologic, geochemical, and tectonic constraints, which combine to indicate that at approximately Chicxulub/K-Pg time a huge pulse of mantle plume-derived magma passed through the crust with little interaction, and erupted to form the most extensive and voluminous lava flows known on Earth. High-precision radioisotopic dating of the main-phase Deccan flood basalt formations may be able either to confirm or reject this hypothesis, which in turn might help determine whether this singular outburst within the Deccan Traps (and possibly volcanic eruptions worldwide) contributed significantly to the K-Pg extinction.

Chronology of the volcanic episodes is improving and pointing to a scenario wherein the volcanism coincided (or was causally connected) to within a hundred thousand years of the Chicxulub impact and overlapped the stratigraphic horizon defined as the Cretaceous -Paleogene boundary. Looks like everyone is going to be partially right on this one.

Tuesday, October 14, 2014

Every now and then there appears a news story about metazoan fossil findings that expresses great astonishment and surprise that there is NOW... THIS TIME.. new evidence that multicellular animals evolved long before their celebrated preservation in the Chengjiang and Burgess shale Lagerstatte.

But we have known that for a long time. The Neo-Proterozoic and early Cambrian fossil record is so much better and is improving and paleo-biologists and palaeontologists have recognized in it the gradual increase in complexity of metazoans over a 50-60 million year period before the exceptional preservation windows of Chengjiang and Burgess shale gives us a false impression of a sudden appearance of complex multicellular animals. This artifact has been exploited by creationists who claim that the fossil record actually supports their creation story of a sudden origin, under some intelligent guidance, of complex animals in the Cambrian, summarized in books like Darwin’s Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design. The best rebuttal I have come across of the creationists many misunderstandings of early animal evolution is this excellent article by Nick Matzke.

Let me just post this invaluable figure below which summarizes the Neo-Protoerozic - Cambrian metazoan fossil record. This is from James Valentine's book On The Origin Of Phyla. It shows clearly that metazoan complexity and diversity increased gradually over time. Molecular phylogeny which aims to reconstruct the last common ancestor of animals based on genetic similarities and differences also tells us that the origin of multicellular animals goes back at least 600 million years ago, maybe even more, a good 80-100 million years before the evolution of calcium carbonate skeletonization made their existence obvious in early Cambrian. Next time a news item appears that claims that somehow fossil embryos or fossil burrows from the Neo-Proterozoic times are some shocking new finding that will change our understanding of animal evolution - don't believe it.

The work is eerily similar to what I did for my PhD which was carrying out a detailed study of cementation patterns in Middle and Late Ordovician carbonate parasequences from the southern Appalachians.

Dickson and Kenter use petrographic techniques along with cathodoluminescence to tease apart the cementation sequence and pore space modification of the carbonate rocks. Hydrocarbon reservoir quality depends in part on how reaction of sediment with water either dissolves material to create pore space or precipitates cements to modify pore space. So, understanding the timing of these events in the context of the burial history of the sediment pile on a basin wide scale can help geologists predict reservoir quality.

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ABOUT THIS BLOG

I am a Sedimentary Geologist. On Rapid Uplift I write mostly about topics within the geosciences, but sometimes on biological evolution and environmental issues. I like to travel and in my free time I teach 12 year old kids soccer and rugby.